Automatic frequency control receiver

ABSTRACT

The invention relates to a receiver of signals [S] received from a wireless network, said receiver working at a so-called reference oscillation frequency controlled by a so-called reference value [Vref]. Said receiver includes demodulation means [DEMO] for demodulating the received signal [S], means [EST] of estimating a mean value [MV] of the demodulated signal [SD], means [COR] of correcting the mean value [MV] of the demodulated signal [SD] to the reference value [Vref], decision means [DEC] for determining the binary values adopted by the received signal [S]. According to the invention, the estimation means [EST] include first means [ESTA] of fast extraction of a first mean value [MVA] of the demodulated signal [SD] used in decision means [DEC] during a first time period and second means [ESTB] of slow extraction of a second mean value [MVB] of the demodulated signal [SD] used in correction means [COR] and, during a second time period, in decision means [DEC].

The invention relates to a wireless receiver and in particular theinvention relates to the elimination of the frequency differencenormally observed on signals received by a wireless receiver between thelocal frequency of the receiver and that of the received signal. Forexample, in the BlueTooth standard, the receivers must be capable ofdealing with large frequency differences. In a wireless system includinga transmitter communicating with the receiver there is generally adifference between the frequency of the transmitter and the frequency atwhich the local oscillator of the receiver works, referred to as thereference frequency. This difference in frequency is manifested as adifference voltage at the output of the demodulator of the receiver.There already exist methods for estimating the difference voltage froman analysis of the demodulated signal over a certain period of time andthen eliminating the difference voltage from the signal, or methodsusing this voltage difference for adjusting the local oscillator in anautomatic frequency control (AFC) loop. Therefore in the prior artwireless receivers working at a so-called reference oscillationfrequency controlled by means of a so-called reference value includedemodulation means for demodulating the received signal, means ofestimating a mean value of the demodulated signal, means of correctingthe mean value of the demodulated signal to the reference value, anddecision means for determining the binary values taken by the receivedsignal.

The function of correction of the mean value of the demodulated signalby an automatic frequency control loop is therefore known. Within thisloop, the estimation means for estimating the voltage difference can,for example, consist of making the demodulated signal pass through alow-pass filter with a narrow passband which eliminates the usefulsignal and preserves the continuous component.

The invention relates to the following considerations:

The choice of the passband for the low-pass filter is a compromise; witha narrow passband this method is slow for estimating the continuouscomponent, and if the passband is widened in order to increase the speedof the operation, the estimation of the continuous component may becorrupted by components of the signal passing through the filter. Thedisadvantage of a slow method is that a long portion of the signal isrequired for estimating the voltage difference before the reception ofthe useful information, and this gives rise to a loss of time or a lossof precision in the detection for each start of a receiving range. Thedisadvantage of a more rapid but more corrupted estimation is thaterrors are introduced into the received signal by the method ofeliminating the difference voltage.

One object of the invention is to make it possible to obtain a receiverfor signals received over a wireless network and, generally, byreceiving ranges, not having the drawbacks disclosed above of the priorart.

For this purpose a wireless receiver in accordance with the introductoryparagraph is characterized according to the invention in that theestimation means include first means of rapid extraction of a first meanvalue of the demodulated signal used in decision means for a firstperiod of time and second means of slow extraction of a second meanvalue of the demodulated signal used in correction means and, for asecond period of time, in decision means.

The two kinds of means of estimating the mean value are complementaryand make it possible not to have either a slow system in which a longportion of the signal is required for estimation of the mean value orerrors introduced into the signal by a faulty estimation of the meanvalue of the signal. The signal reception circuit obtained thereforecombines slow and fast estimations of the frequency so as tosimultaneously achieve a precise correction of the frequency and goodsensitivity of the receiver from the start of the receiving range.

In a particular embodiment of the invention, the means of correcting themean value of the demodulated signal use a frequency correction loop.The principle of automatic frequency control is known for analog dataand for digital data.

In an advantageous embodiment of the invention, the first estimationmeans include means for evaluating the minimum value and the maximumvalue of the received signal and thus estimating the mean value of thesignal at the median value between these two values. Such an estimationis very rapid. However, it suffers from relative imprecision (forexample, when a series of consecutive ones or zeros are present in thesignal). This estimation could be used for the entire signal but theprecision of the decision means could be affected thereby.

Thus, in a preferred embodiment of the invention, the received signalconsisting of a synchronization and control part and then a data part,the first period does not exceed the period necessary for the receptionof the synchronization and control parts.

The invention can be implemented in any receiver working on a wirelessnetwork in which significant frequency differences may arise. BlueTooth,DECT etc technologies and any other technology presenting thecharacteristics disclosed above are thus concerned.

The principle according to the invention can just as well be used fordigital data or analog data.

The invention also relates to an integrated circuit in which there is areceiver according to the invention.

The invention also relates to a method of receiving and processingsignals received from a wireless network according to the invention.

The invention will be further described with reference to examples ofembodiments shown in the drawings to which, however, the invention isnot restricted.

FIG. 1 is a block diagram of a receiver according to the invention,

FIG. 2 is a block diagram of the general principle of an automaticfrequency control loop,

FIG. 3 is an example of an embodiment for an amplifier implementedwithin a frequency control loop,

FIG. 4 is a functional diagram of a receiver according to the inventionaccording to an advantageous embodiment,

FIG. 5 is a time diagram for controlling the various elements of theinvention according to a preferred embodiment of the invention,

FIG. 6 presents the result of the frequency correction obtained with theimplementation of the invention by means of a set of curves representingvoltages at various points in the circuit of FIG. 4.

The following remarks relate to the reference signs. Similar entitiesare designated by identical letters in all the figures. Several similarentities may appear in a single FIGURE. In this case, a number or suffixis added to the reference by letters in order to distinguish similarentities. The number or suffix may be omitted for reasons ofconvenience. This applies to the description and to the claims.

The description which follows is presented so as to enable a personskilled in the art to implement and make use of the invention. Thisdescription is provided in the context of the patent application and itsrequirements. Various alternatives to the preferred embodiment will beobvious to a person skilled in the art and the generic principles of theinvention disclosed here may be applied to other implementations.

Thus the present invention is not deemed to be limited to the embodimentdescribed but rather to have the broadest scope in accordance with theprinciples and characteristics described below.

FIG. 1 presents a block diagram of a receiver according to theinvention. This figure presents means for performing various functionsnecessary for implementing the invention. These functions are steps in areception method according to the invention. A receiver according to theinvention receives signals S from a wireless network. The receiverincludes reception means, not shown, which may for example be anantenna. The receiver works at a so-called reference oscillationfrequency, the said oscillation frequency being controlled by means of aso-called reference value Vref. This reference value is generally avoltage. A receiver according to the invention includes demodulationmeans DEMO for demodulating the received signal S, means EST ofestimating the mean value MV of the demodulated signal SD, means COR ofcorrecting the mean value MV of the demodulated signal SD to thereference value Vref, and decision means DEC for determining the binaryvalues adopted by the received signal S. According to the invention, themeans EST of estimating the mean value of the received signal includefirst means ESTA of rapid extraction of a first mean value MVA of thedemodulated signal SD used in decision means DEC during a first periodof time and second means ESTB of slow extraction of a mean value MVB ofthe demodulated signal SD used in correction means COR and, during asecond period of time, in decision means DEC. The data manipulated bythe receiver according to the invention can be digital or analog.Hereinafter, an application of the invention to an analog processing ofdigital data is presented. A means of estimating the mean value willthen be means intended for the processing of analog data. Digital meansof estimating digital data can also be used without detriment to theprinciple of the invention.

In a particular embodiment, and according to what is presented in FIG.1, the means COR of correcting the mean value MV of the demodulatedsignal SD are looped onto the demodulator DEMO, thus forming what iscommonly referred to as a frequency correction loop. Such a loop isgenerally known by the term automatic frequency control. FIG. 2 presentsa block diagram of the general principle of an automatic frequencycontrol loop. The function of such a frequency control loop is two-fold.It keeps the frequency fIF constant whatever the frequency difference inthe received signal. It also compensates for any variations in methodand temperature by automatically modifying the frequency fIF to thecenter frequency of the demodulator. This is achieved by looping theinformation on the mean frequency fIF onto the local oscillator VCO ofthe receiver so that the adjustment of the local frequency fVCO thereoftakes place automatically. The output signal of the demodulator DEMO isfiltered in a low-pass filter FIL in order to extract the mean value ofthe signal frequency. This filter FIL therefore constitutes means ESTBof slow estimation of the mean value MVB of the demodulated signal SD.An example of such a filter will be given in the description of FIG. 4.The mean value MVB is then compared with a reference value Vref. Thiscreates, for example, an error signal. This information is amplified andapplied to a control input of the local oscillator VCO. This results ina change in the frequency of the VCO which tends to cancel out thefrequency difference between the signal and the local oscillation.

More precisely, the demodulator is assumed to have a linear response inthe following form:SD=Vc−Kd(fIF−fc)

-   -   where Vc is the output voltage when fIF is equal to fC and Kd is        the demodulator gain. A voltage SD is therefore available at the        output of the demodulator. The demodulator DEMO is followed by        means ESTB of estimating a mean value MVB of the signal SD.        These estimation means ESTB advantageously consist of low-pass        filtering means FIL as seen above. The mean value MVB obtained        is then proportional to the difference in frequency between the        frequencies fIF and fc. Next the mean value MVB is compared with        a reference value, which is generally a voltage Vref, and        amplified by amplification means AMP of gain A and then supplied        to the local oscillator of the receiver VCO. This is summarized        in the following expression:        Vmod=Vref+A(Vin−Vref)

The output signal of the amplifier AMP is applied to the localoscillator VCO functioning at the frequency fIF. Thus:fVCO=fref+Km(Vmod−Vref)

fref is the frequency when Vmod=Vref and Km is the modulation gain. Eachdifference in the frequency of the signal therefore modifies the currentfrequency fIF of the same value but in an opposite direction, and:fIF=fVCO−ΔfRF

Next the response of the system to a variation in the frequency RF maybe a difference in the carrier frequency or the modulation itself.

Using the previous equations, the following expression is finallyobtained:${{fIF} - {fref}} = \frac{{{KmAF}( {{Vc} - {Vref}} )} + {{KmAFDKd}( {{fc} - {fref}} )} - {\Delta\quad{fRF}}}{1 + {KmAFKd}}$

If Vc=Vref, the equation is simplified as:${{fIF} - {fref}} = \frac{{{KmAFKd}( {{fc} - {fref}} )} - {\Delta\quad{fRF}}}{1 + {KmAFKd}}$

Ideally the system is constructed so that fc=fref. In other words, thismeans that, when fIF=fref, the output voltage Vc=Vref. Because ofvariations in the method for example, this cannot be the case. When thefrequency difference of the signal ΔfRF is zero and if the gain of theloop KmAFKd is fairly large, the equation shows that the loop recentersthe frequency fIF towards fIF=fc.

In the case of a frequency difference in the received signal, the loopkeeps the frequency fIF close to fc, since the voltage difference isattenuated by a factor 1+KmAFKd.

The loop must compensate for any slow drifts in frequency of thereceived signal but not react to its modulation. This is achieved byoptimization of the loop filter.

By choosing a first-order filter with the transfer function:${F(p)} = \frac{1}{1 + {\tau\quad p}}$

For fc=fref, there is obtained:${{fIF}(p)} = {{fref} + {\frac{1 + {\tau\quad p}}{1 + \frac{\tau\quad p}{1 + {Gloop}}} \cdot \frac{\Delta\quad{{fRF}(p)}}{1 + {Gloop}}}}$

-   -   where Gloop=KmAF(0)Kd is the static loop gain.

For a received signal varying rapidly so that${\frac{\tau\quad p}{1 + {Gloop}} ⪢ 1},$the loop does not react, and the modulation is thus transferred withoutattenuation in the loop. The loop amplifier must have a high inputimpedance in order to be sensitive to the voltage present on the loopfilter capacitor. Its output controls the tuning of the local oscillatorVCO. The amplifier is for example implemented according to FIG. 3 by anamplifier of the transconductance type with returns.

The loop amplifier can be broken down into a gain term Gv and a returngain term β with:${Gv} = {{gm}\frac{{Rout}( {{R1} + {R2}} )}{{Rout} + {R1} + {R2}}}$$\beta = \frac{R1}{{R1} + {R2}}$

The closed loop gain is:$\frac{{Vout} - {Vref}}{{Vin} - {Vref}} = \frac{Gv}{1 + {\beta\quad{Gv}}}$

For example, in the application a gain of approximately 20 is required.As the transconductance is an amplifier with one stage, the gain is notvery great, and the gain aimed at is achieved with a fairly low return(Gvβ slightly greater than unity). We have seen here the generalfunctioning of an automatic frequency control loop. In the applicationto a receiver functioning on a wireless network such as, for example, aBlueTooth receiver, decision means DEC intended to determine whether thesignal corresponds to a one (1) or to a zero (0), that is to say todetermine the binary value of the received signal, are necessary. Theautomatic frequency control loop and the said decision means DEC requireextraction of the mean value of the signal but have differentrequirements with regard to this extraction. For the decision means DECthe extraction must above all be rapid during the start of the receivingranges of the receiver. This start of the receiving ranges generallycorresponds to an access code and to signal synchronization and errorcontrol signals. On the other hand, for the frequency control loop, aslow extraction is permanently required in order to prevent attenuationor cancellation of the modulation by the frequency control loop. Duringthe start of the receiving range, the extraction of the continuouscomponents is preferably performed by first means of estimating the meanvalue ESTA. Thus the loop can have a low gain (6 for example). Theperiods during which the extraction is performed by the various meansEST of estimating the mean value MV are chosen according to thecharacteristics of the signals intended to be received by the receiver.According to FIG. 4, these periods are controlled by control means,which are switches controlled according to a time diagram as for exampledisclosed in FIG. 5.

FIG. 4 presents an advantageous embodiment of the invention. The meanspresented in FIG. 1 are found again in more detail and in the context ofan analog processing of the data.

According to FIG. 4, the receiver receives the signal S in itsdemodulator DEMO. The demodulator actuates a switch R_ON in order to putthe system in a state of receiving data. Once the demodulation has beenperformed, the demodulated signal SD is supplied as an input to twokinds of means ESTA and ESTB of estimating the mean value MVA and MVB.The first means ESTA, which are rapid, perform for example an evaluationof the minimum and maximum values of the signal and then take, as a meanvalue of the demodulated signal SD, the median value between these twovalues. One implementation of such an evaluation is depicted in FIG. 4.The evaluation of the extremum values of a signal and the calculation ofthe median value, in particular as described in FIG. 4, are well knownfrom the prior art and will therefore not be described in any furtherdetail here. The second estimation means ESTB consist for example,according to FIG. 4, of a low-pass filter R2C1. The value of thecapacitance C1 is chosen from a compromise on the time constant of theRC filter referred to as ESTB. This value is generally high and thecapacitor is therefore external. The time constant must be fairly longin order to obtain a good estimation of the mean value of the outputsignal of the demodulator but fairly short in order to ensure that thelevel of the filter output is already close to its final value at theend of the access code, when the reference input of the decision meansis switched to the output of the slow RC filter. It is remarkable tonote that the invention makes it possible to have only one externalcapacitor in the circuit, which represents a saving in space andmanufacturing time for the receiver. This single external capacity isused for the frequency control loop permanently and by intervals of timefor the decision means.

The correction means include, in the same way as presented in FIG. 2, anamplifier AFC connected to an input of the local oscillator VCO, thewhole being put in a loop with the demodulator DEMO. The decision meansDEC include an amplifying element SLI which fixes the value of theoutput at 1 or at 0 according to the value on two inputs, the firstreceiving the mean value of the signal coming from one of the estimationmeans ESTA and ESTB and the second receiving the demodulation signal SDitself. Advantageously, this amplifying element SLI also deactivates theswitch signal R_ON activated by the demodulator DEMO when there is nolonger any signal to be processed. According to a preferred embodiment,the various switches are actuated according to the time diagramspresented in FIG. 5.

Before the receiving range begins, that is to say, in FIG. 5, on thepowering up of the receiver, the external capacitor C1 is precharged tothe reference voltage Vref by activating the switch S_EN, which appearson the first line of the time diagram in FIG. 5. When the receiver ispowered up, the local oscillator VCO and the loop PLL are also startedup. At the start of the receiving range, the detection of a signal isindicated to all the elements of the receiver by the signal R_ONtriggered by the demodulator DEMO. The reception REC is then commencedand the data DATA arrive in the form shown for example in FIG. 5. Thesedata, which constitute the received signal, are formed by an access codeconsisting of several synchronization and error check elements PR, SYN,TR, and then a packet of specific data PAYL, which are the data peculiarto the communication next processed by the receiver. The switch R_ONthen makes the external capacitor C1 switch onto the output of thedemodulator DEMO for extraction of the mean value MVB of the signal bythe second estimation means ESTB. A relatively long time constant isused. In the case of a frequency difference, the mean value of thesignal MVB is different from the reference voltage Vref. Thisinformation is supplied to an input of the local oscillator VCO in orderto correct the frequency of the receiver. This forms what is known as anautomatic frequency control loop. During this time, the receiver mustdetermine whether the received bits are zeros or ones. This must be doneby comparison with, also, a mean value MV of the signal. The mean valueMVB supplied by the estimation means ESTB cannot be used at the start ofthe receiving range since it does not converge sufficiently quicklytowards the actual mean value. Another circuit is therefore used. Thiscircuit includes the estimation means ESTA, which determine the meanvalue from a detection of peaks. These estimation means ESTA arereinitialized by the activation of the switches SR. Then the detectionof the peaks and the evaluation of the values of peaks MIMA areperformed during a time constant illustrated in FIG. 5. During thisevaluation, the switch S2 is activated whilst S3 is open, so that themean value of the signal is available at the output of the estimationmeans ESTA and available at the reference input of the determinationelement SLI. Next, by switching the switch S2 into the open position andthe switch S3 into the closed position, the reference input for thedecision means DEC is switched to the mean value MVB measured by theestimation means ESTB. Then the frequency control loop and the decisionmeans use the same mean value of the signal MVB. The time constant ischosen so that the switching takes place before the arrival of the datapacket PAYL including the particular data of the communication nextprocessed by the receiver.

A simulation was performed with the values put forward in the tablebelow for the various elements in FIG. 4. This simulation is put forwardin FIG. 6. R1 R2 RB RC RD C1 CB CI 0.5K 50K 4M 2K 100K 8 nF 10 pF 22 pF

These values are given by way of indication and are not given to theexclusion of other values or range of values. For example, the externalcapacitance C1 can be increased if, for a specific application, the timeconstant for acquisition of the data and adjustment of the mean value tothe reference value can be increased. Simulations were performed, forexample, for a carrier frequency difference of around a hundred kHz (200kHz in the example in FIG. 6). In such simulations, for example, theactions performed on the time diagram in FIG. 5 are found. The loop PLLand the frequency control loop are activated first. The frequencycontrol loop is initialized with the same reference voltage Vref, forexample 1.45 V, as the one which is used for the local oscillator. Afterthe loop PLL has been established at the required frequency, thereceiver is activated, in accordance with the diagram in FIG. 5, at 200μs. The data arrive at 225 μs on the curve Demod_OUT. It should be notedin FIG. 6 that these data have a difference with respect to thereference value Vref charged on the external capacitor C1 and depictedon the curve C ext. By virtue of the invention, on the simulation, theeffect of the frequency control loop which restore the data andtherefore the curve Demod_OUT demodulated around Vref, represented bythe curve C ext (Vin AFC), is then observed. The mean voltage used bythe decision means depicted on the curve DC_Slicer is first of allsupplied by the extraction means by the detection of peaks on the curveDemod_OUT, which operate on the access code and the header packet of thedata packet up to approximately 350 μs according to FIG. 5 and in FIG.6. At this moment, the frequency control loop has corrected the initialdifference. The level of the continuous component MVA extracted by theextraction means by detection of peaks ESTA is close to the mean levelindicated by the external capacitance C1 MVB. Then the reference inputof the decision means is switched to the external capacitor C1.

Although this invention has been described in accordance with theembodiments presented, a person skilled in the art will immediatelyrecognize that there exist variants to the embodiments presented andthat these variants remain in the spirit and within the scope of thepresent invention. Thus many modifications can be made by a personskilled in the art without for all that being excluded from the spiritand scope defined by the following claims.

1. A receiver for signals received on a wireless network, said receiverworking at a so-called reference oscillation frequency, said oscillationfrequency being controlled by means of a so-called reference value,including: demodulation means for demodulating the received signal,means of estimating a mean value of the demodulated signal, means ofcorrecting the mean value of the demodulated signal to the referencevalue, decision means for determining the binary values adopted by thereceived signal, characterized in that the estimation means includefirst means of rapid extraction of a first mean value of the demodulatedsignal used in decision means during a first period of time and secondmeans of slow extraction of a second mean value of the demodulatedsignal used in correction means and, during a second period of time, indecision means.
 2. A receiver as claimed in claim 1, characterized inthat the means of correcting the mean value of the demodulated signaluse a frequency correction loop.
 3. A receiver as claimed in claim 1,characterized in that the second extraction means include low-passfiltration means for extracting the mean frequency of the signal.
 4. Areceiver as claimed in claim 1, characterized in that the firstestimation means include means for evaluating the minimum and maximum ofthe received signal and estimating the mean value of the signal at themedian value of the minimum and maximum.
 5. A receiver as claimed inclaim 1, characterized in that, the received signal consisting of asynchronization and control part and then a data part, the first perioddoes not exceed the duration necessary for receiving the synchronizationand control parts.
 6. An integrated circuit including a receiver asclaimed in claim
 1. 7. A method of receiving and processing signalsreceived on a wireless network, including the steps of demodulation fordemodulating the received signal, estimation of the mean value of thedemodulated signal, correction of the mean value of the demodulatedsignal to the reference value, decision for determining the binary valuetaken by the received signal, characterized in that the estimation stepincludes a rapid substep of extracting a first mean value of thedemodulated signal used in the decision step during a first period oftime and a second substep of slow extraction of a second mean value ofthe demodulated signal used in the correction step and, during a secondperiod of time, in the decision step.